Wireless communication systems including a plurality of wireless terminals constituting a wireless ad hoc network have been known. In this type of wireless communication systems, each wireless terminal is mostly configured to perform wireless communication with another wireless terminal according to a Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) scheme.
In the CSMA/CA scheme, each wireless terminal performs carrier sense before performing communication. The carrier sense is a process of detecting whether or not there is a radio signal transmitted from another wireless terminal through a frequency used to perform wireless communication. Then, each wireless terminal performs wireless communication through the frequency when there is no radio signal transmitted from another wireless terminal. For example, the CSMA/CA scheme is employed in IEEE802.11.
Meanwhile, it is assumed that there are a second wireless terminal and a third wireless terminal which are able to communicate with a first wireless terminal, and the second wireless terminal and the third wireless terminal are unable to communicate with each other. For example, when an obstacle is present between the second wireless terminal and the third wireless terminal, the above-mentioned situation may occur.
In this case, when the third wireless terminal performs the carrier sense while the second wireless terminal is in the process of transmitting a radio signal to the first wireless terminal, it is difficult for the third wireless terminal to detect the presence of the radio signal transmitted from the second wireless terminal. For this reason, the third wireless terminal also starts to transmit a radio signal to the first wireless terminal. As a result, a collision of the radio signals occurs in the first wireless terminal, and the quality of the radio signal received by the first wireless terminal deteriorates.
This problem is referred to as a hidden terminal problem. The hidden terminal problem may cause communication efficiency to degrade. Particularly, in a wireless communication system in which a certain wireless terminal (receiving terminal) receives information from a relatively large number of wireless terminals (transmitting terminals), since a collision of the radio signals frequently occurs in the receiving terminal, the communication efficiency is likely to degrade due to the hidden terminal problem.
In order to solve this problem, first to third techniques have been known (for example, see Japanese Laid-open Patent Publication No. 2008-85924, 2006-5812, 2006-197483, and 2008-227854). The first technique is a technique of determining a communication path based on received signal strength indication (RSSI) of a radio signal transmitted from another wireless terminal.
The second technique is a technique in which each wireless terminal transmits a list of wireless terminals which are able to communicate with its own terminal to a wireless base station, and the wireless base station divides the wireless terminals so that a collision of radio signals does not occur into groups based on the list and performs transmission control for each group. The third technique is a technique in which each wireless terminal obtains a parameter representing a communication quality for each wireless terminal which is able to communicate with its own terminal, and determines a communication path based on the obtained parameter.
Meanwhile, it is assumed that there are a second wireless terminal, a third wireless terminal, and a fourth wireless terminal which are able to communicate with a first wireless terminal, the second wireless terminal is unable to communicate with the third wireless terminal and the fourth wireless terminal, and the third wireless terminal and the fourth wireless terminal are able to communicate with each other.
In this case, among wireless terminals which are able to communicate directly with the first wireless terminal, a hidden terminal number which is the number of wireless terminals which are unable to communicate directly with the second wireless terminal is 2. Meanwhile, among wireless terminals which are able to communicate directly with the first wireless terminal, a hidden terminal number which is the number of wireless terminals which are unable to communicate directly with the third wireless terminal is 1. Thus, the hidden terminal number for the second wireless terminal is different from the hidden terminal number for the third wireless terminal.
However, when the second wireless terminal and the third wireless terminal are located at the same position from the first wireless terminal, the second wireless terminal and the third wireless terminal are the same in the RSSI of a radio signal transmitted from the first wireless terminal. That is, there are cases in which the RSSI does not reflect the hidden terminal number. Thus, when a communication path is determined based on the RSSI, a collision of radio signals is likely to frequently occur in the first wireless terminal (receiving terminal).
Further, when each wireless terminal determines a communication path based on information (for example, an identifier identifying a wireless terminal or a parameter representing a communication quality) of each wireless terminal capable of communicating with its own terminal, a load (for example, a processing load needed to determine a communication path or a communication load needed to communicate the information) is likely to excessively increase.
As described above, in the related arts, there is a problem in that it is difficult to increase communication efficiency without an excessive increase in a load.